Foundation Edition of Windows Server does not use Client Access Licenses (CALs) - sort of. Foundation Edition is limited to 15 users - PERIOD. You cannot add more. You can view foundation as including the CALs or not needing them... in either case, 15 users is it.
Client Access Licenses in general are use to grant users (or devices) the right to access the server. Typical Server CALs cover authentication and access to most included features of Windows Server. One noteable exception is Remote Desktop Services (RDS) a.k.a. Terminal Services. These require special, separate CALs that are considerably more expensive. Then there are Small Business Server CALs which are exclusively for use on networks running Microsoft's Small Business Server (SBS) product (Foundation is NOT SBS).
This can be complicated, but it can be generally answered as well. To ensure you don't purchase the wrong item(s), I agree, you need to contact a consultant who can guide you. It's amazing the number of companies I walk into (and people I find posting in forums) who have SERIOUSLY OVERSPENT trying to "save" by not hiring a consultant. 2 hours of consultant time at $150 per hour can save THOUSANDS on the purchase price, lost productivity, and poor performance.